You can buy a vacuum leak test kit from Harbor Freight. Seal off the carb flange with a flat plate and a gasket, then use one of the vacuum ports in the intake to use in drawing a vacuum on the whole intake, after loosening the rockers from the intake valves like you mentioned before. The kit comes with a vacuum gauge I think (my son bought me one but I've yet to open it up to use it) Once you pull a vacuum, stop and watch the gauge to see if there's a leak. Same thing can be done with a pressure test, but that would tend to force the intake valves off their seats. If you're concerned about your hand getting tired pumping, by a small electric pump to pull the vacuum.
Do I understand that the vac test is for checking valve seal ? I thought that the cranking compression and leak down tests had ruled out valve leakage. I am thinking that I can loosen the intake rockers of the open valves, and that would isolate the intake manifold from the cylinders. Then if I got smoke through the crank case, that would confirm a bad seal between the intake and head. The last smoke test was done with the carb in place and capped with a small stainless mixing bowl. Some intake valves were open so I think that test was invalid. I have a venturi vac pump so if the closed valve smoke test is inconclusive, I will try that. Last time I just blew the smoke in through the large manifold vac port on the front of the throttle body. Not much pressure and not much resistance. I'm going to go try it now.
You can use a vacuum test for many things. Using a vacuum gauge on the intake manifold after pulling a vacuum on it with the intake valves closed would let you see by watching the gauge if there's a leak. You'd need to install a Tee on the manifold for the gauge and a shut off valve to isolate the pump from the manifold as well to do this. Same as you would doing a pressure test. Only difference is a pressure test might force the intake valves open.
jbrich, if I were you I'd just replace the RPM intake with one of your known quality manifolds and see how things work out. An intake swap is not a big deal on a small block Ford. No distributor to pull! You use RTV on the intake ends not gaskets, right?
That's exactly what I would do at this point. Then fully inspect all machined surfaces for imperfections(which cheap gaskets struggle to seal without gasket dressings) that may have been missed during quality control or the casting simply moved around since manufacture. Check heads and manifold flatness, straight and criss-crossed corner to corner of bot mounting flanges. Shine a flashlight behind a straightedge, doesn't require a machinists square a good level will work, with a good eye sometimes feeler gauges are not even required. From there you'll need to mock the clean bare intake back up between the heads and torque very lightly to set it snug for measuring. Sometimes it's tough to see the flashlight from end to end and get solid measurements so I generally prefer to shim all 4 corners a little more than compressed gasket thickness. Btw, small squares of 16 gauge masking taped to the head are almost right at .060" thick) that are placed away from the upper and lower portions of each flange, more towards the middle to be able to better see a flashlight from end to end. If somethings really tweaked you will not even need to do much more than dry mock it to see the uneven mismatched gaps. Installed Ford heads are milled at 45 degree manifold inclination, 90 degrees off heads fire deck so proper block and head milling shouldn't affect it like many other motors which need to be corrected. Another thing I just thought of too(after being reminded by a few other negative comments) is that Edelbrock sometimes has a terrible quality control process and there are often blind holes which have been drilled all the way through the carbs mounting flange. Couple years back returned a Cleveland RPM air-gap because of that(3 out of 4 possible were needlessly drilled into intake ports) and some other quality control issues. My current 351W air-gap also has one drilled through into the upper plenum runner that will soon be welded shut. I'm not sure what length carb mounting studs they think we're using out here in the real world but the fully threaded hole was over 1 1/2" deep to actually punch through the ports roof. Also worth noting that many have had cylinder heads(stock and aftermarket) where the rocker studs are drilled through some but not all intake port roofs and thread sealer needs to be used to prevent small intake leaks and oil draw into some cylinders. Easy to check for oil residue around or inside that hole in the intake ports while you have it apart. Small piece of paper towel with little twisted hook on the end to stick up into the rocker stud hole will sometimes pick up fresh clean wet oil droplets to clue into the problem. If everything checks out and you reassemble without finding the issues I would highly advise some quick-tack sprayed on the head side of the gasket along with VERY small amounts of RTV finger smeared around each water port. RTV or non-haredening aviation sealer works decent and easier to clean off the manifold side of the gasket. Another good one, messy to disassemble but good, is Hylomar dressing. I'd even use it very sparingly on all the carb/manifold gasket interfaces. Seal it up so well that it's nearly impossible to leak for process of elimination. Are you also sure the carb's throttle shafts aren't worn and leaking? If they're bad enough to cause what you describe it should be audible when you get close enough. Sometimes you can even hear them crackle unevenly or inconsistently as the engine idles through its firing order. Higher idle speeds will make it more consistent and pronounced. Can also wiggle and side load the shafts to hear rpm or leakage sound changes. Good luck in the hunt!
I think I am down to swapping the manifold. I didn't pressure or vac test it yesterday because I decided to set the advance curve on the procomp distributor real quick. Easier said than done. After wasting too much time on that 'THING' I swapped back to the DS system so that I can put my air cleaner back on. I do use just rtv to seal the ends. I used Permatex high temp copper spray gasket sealer on both sides of the manifold gaskets. I've never used Hylomar but I'm going to get some. It sounds like good stuff. If it's good enough for Rolls Royce it's good enough for me. I did suspect the driver side of the secondary throttle shaft. When I sprayed ether directly on the shaft it was drawn in and it changed the idle. I packed the shaft with high pressure grease and it didn't change the idle but it sealed around the shaft and prevented the ether from being drawn in. Wiggling the shaft didn't change anything. I saw on one of the car shows they used a couple of pieces of lead split shot placed high and low between the head and manifold. They dry torqued it with gaskets then pulled it of and measured the shot for comparison. Tonight I'm going to thoroughly inspect and paint the manifold I choose. I have a Weiand 2p 180 and the performer 289. They have different runner layouts. I don't know if that makes a difference in performance. Does anyone have advice on the difference between these manifolds ?
Hi Frank, I'm pretty sure ( 70 to 80 % ) that there are no holes or cracks. I think I would have seen something like that while prepping for paint. But I didn't specifically look for flaws. I could have missed something. Give me your ideas. I'm still going to pressure/smoke test the intake. As soon as it gets over 40 degrees outside.
Plan b sounds good. Pardon my ignorance but what exactly is plan b ? Keep in mind that I am not wanting to do an LS swap at this time. I am open for any ideas.
Do you believe the problem is in the primary side? Or is it in the secondary side ? How would I troubleshoot this down to the component ?
Threads get long and muddled but seems like you already mentioned making it most if not all the way through that troubleshooting tree? Did you miss or skip something? Ideally you test and then swap parts out with known good ones one component at a time for process of elimination. Have you done the basic "cylinder kill" process whereby you simply pull plug wires on the rough idling motor to differentiate which cylinder/s are actually dead or missing? Even just swapping wires around can sometimes suss out bad or intermittent connections. Assuming no major angle mismatches or warpage between heads and intake, I think something was missed in the ignition system.. or.. the spark is weaker than indicated in the actual running cylinder. Free air spark intensity is sometimes not much of a final confirmation towards the true reality of energy output within compressed charges. Coil and/or voltage related issues can be really erratic and inconsistent sometimes.. causing tons of threads just like this one here and everywhere else on the net. Even lots of seasoned mechanics have chased their tails and tossed parts at figuring those sorta things out. Occasionally, bad/weak or lack of sufficient engine ground paths can freak electronics out and commences the tail chasing. IMO, the size of the intake leak required to make the motor run as you describe should be large enough to have been pinpointed by now. And bare fitting THE or any other manifold between the heads should make fitment and potential sealing issues readily apparent. Temporary and very liberal amounts of gasket dressing can seal up some pretty bad leaks to help eliminate potential causes. Still seems to be a fueling or spark related issue to me. Almost need a condensed troubleshooting list of things tried and parts swapped out so far to get everyone back up to speed.
I think you found the problem in that carb shaft. The Performer 289 is an inferior intake to the RPM, the difference between the two is the RPM shines in the upper rpms, both are equal in the bottom end, despite what the advertising says. Not sure what the Wieand intake is, their Stealth is the equal to the RPM.
Obviously wouldn't rule it out but wouldn't the carb swap off his Comet have eliminated the shaft leak issue? 2 bad carb's? Something just does not compute here. Who knew it could be so hard to fix a non-computer controlled engine with a computer keyboard? lol
